Lowepro fastpack!! I owned the 250 loved it!!! When i got a 17'' laptop I instantly purchased the 350 at the same time. awesome bag in everyway. only 2 downsides for me verse a 100 positives. No AW cover & no tripod holder, but in no way takes away from it being a fantastic photo backpack.
As for you concern of possible theft from the side opening, I would not worry about it, the zipper actually zips all the way around the bag, so you can stick it under the flap, so someone would have to unbuckle 2 clips, just to get to the zipper, then after undoing the 2 clips, unzipper it while you are wearing it, to get to your camera, all while you dont notice a thing, which I don't feel can happen. Anyway when I travel, I put a luggage lock on that zipper anyway for the peace of mind, cost $2.
Also this bag does not stand out, or seem like a photo backpack in my opinion, especially in all black, so it wont stand out in publc or scream photo equiptment!!!